Exploring hydrophilic 2,2-di(indol-3-yl)ethanamine derivatives against Leishmania infantum

Herein we report the design and the synthesis of a library of new and more hydrophilic bisindole analogues based on our previously identified antileishmanial compound URB1483 that failed the preliminary in vivo test. The novel bisindoles were phenotypically screened for efficacy against Leishmania infantum promastigotes and simultaneously for toxicity on human macrophage-like THP-1 cells. Among the less toxic compounds, eight bisindoles showed IC50 below 10 μM. The most selective compound 1h (selectivity index = 10.1, comparable to miltefosine) and the most potent compound 2c (IC50 = 2.7 μM) were tested for their efficacy on L. infantum intracellular amastigotes. The compounds also demonstrated their efficacy in the in vitro infection model, showing IC50 of 11.1 and 6.8 μM for 1h and 2c, respectively. Moreover, 1h showed a better toxicity profile than the commercial drug miltefosine. For all these reasons, 1h could be a possible new starting point for hydrophilic antileishmanial agents with low cytotoxicity on human macrophage-like cells.


Reviewer #3
Comments: The manuscript (PONE-D-24-01889) entitled 'Exploring hydrophilic 2,2-di(indol-3-yl)ethanamine derivatives against Leishmania Infantum' by prof.Simone Lucarini, Paula Kiuru and coworkers describes the design and synthesis of a new library of hydrophilic bisindole substituted analogues based on the structure of the antileishmanial compound namely URB1483, previously developed by them, which failed on the in vivo evaluation probably due to its low water solubility as the authors mentioned.In addition, the authors provide the biological evaluation of the synthesized compounds concerning against Leishmania infantum promastigotes and simultaneously for their toxicity on human macrophage-like THP-1 cells.The most active compounds 2c and 1h with IC50= 2.7 μM and IC50= 4.1 μM showed comparable activity with the oral drug miltefosine (IC50= 3.6 μM) and also comparable selectivity index only in case of 1h.The toxicity profile of the 1h was slightly better than miltefosine.On the other hand the most active compound compound 2c was more toxic than miltefosine but seems to be promising for further chemical modifications in order to achieve an improved toxicity outcome.
The manuscript overall is well written and the compounds are well characterized but some corrections are needed.Overall, the manuscript merits publication in PLOS ONE as long as minor revision takes place according to the following suggestions: a) I don't fully understand the rationale related with the design of the synthesized series of compounds 2 and 3.More specifically, among the synthesized compounds included in the first series of compounds (free amine bis-indoles), 1a, 1b, 1c, 1f and 1h were found to be active against L. infantum promastigotes in the low micromolar range, with IC50s between 12.9 -4.1μM.Among them 1a have no substitution at the aromatic ring of the NH-Indole moieties, 1b, 1c and 1f are substituted at position 6 of the NH-Indole moieties with a fluorine, chlorine or a methylester respectively and finally 1h bearing a methyl group at the indole nitrogen atoms.Since none of them bearing a 6-bromo substitution at the indole moieties and also taking into consideration that the most active was 1h with no substitution at position 6, in my opinion the authors should provide a more detailed discussion in the manuscript explaining the rationale followed for the design and synthesis of compound series 2 and 3 and mainly should explain why they proceeded only with modifications on the structure of 1d (bearing bromine atoms at 6 position of the indoles).
A: We thank the referee for constructive comments that allow us to improve our manuscript quality.As correctly noticed, the 6-Br bisindole 1d did not show preliminary promising results as 1a-c, 1f or 1h.However, we designed the compounds library before biological test results considering the known biological activity of the privileged scaffold 1d.Indeed, classes 2 and 3 were synthetized from 1d, which is a marine bisindole alkaloid isolated from the californian tunicate Didemnum candidum and the new caledonian sponge Orina spp.
These considerations have been added in the text at page 12, lines 380-384, together with the opportune references 30 and 31.b) In the general procedure A used for the synthesis of bisindoles 1 the authors described that the reaction performed under microwave irradiation.Since it would be very useful for the readership of PLOS ONE, they should provide more details about the reaction condition such as the microwave power (watts) used in all the reactions performed under microwaves.
A: Thanks for the suggestion.Microwave syntheses were performed using the set temperature mode, which foresees to keep the temperature constant and the instrument automatically and consequently adjusting the power.Depending on the chosen temperature (i.e.120°C or 80°C), the microwave power varied from 30 to 75 W.As required by the reviewer, this information has been added.